This invention relates to electromechanical actuators in general and particularly to fast-response electromagnetic valves such as fuel injectors for internal combustion engines. More particularly, this invention relates to fast-response fuel injectors having a dual coil configuration.
Electromagnetic actuators, such as fuel injectors, typically contain solenoids. A solenoid is an insulated conducting wire wound to form a tight helical coil. When current passes through the wire, a magnetic field is generated within the coil in a direction parallel to the axis of the coil. The direction of the magnetic field generated within the coil depends on the direction of the current passing through the wire as well as the direction in which the wire is wound (e.g., clockwise or counter-clockwise). When the coil is energized, the resulting magnetic field exerts a force on a moveable ferromagnetic armature located within the coil, thereby causing the armature to move from a first position to a second position in opposition to a force generated by a return spring. The force exerted on the armature is proportional to the strength of the magnetic field; the strength of the magnetic field depends on the number of turns of the coil and the amount of current passing through the coil.
There are typically three phases in a fuel injector cycle: the opening phase, the hold-open phase, and the closing phase. For reasons of efficiency and performance, it is desirable to have the opening and closing phases be as fast as possible. It is also desirable to control the current through the injector coils in all phases of the injector cycle such that the amount of energy dissipated within an injector in the form of heat is minimized. Typically, during the opening phase, the magnetic field is required to build as rapidly as possible to minimize the opening time. Because the hold-open phase requires much less force than the opening phase, during the hold-open phase the magnetic field strength should be reduced to the minimum level sufficient to ensure the valve will remain open until the closing phase is initiated by the engine control unit (ECU). By selecting the minimum necessary hold current level during the hold-open phase, the decay time of the magnetic field from the hold-open level to the xe2x80x9cbreak awayxe2x80x9d level will be minimized. The xe2x80x9cbreak awayxe2x80x9d level is the magnetic field strength at which the armature separates from the pole piece and mechanical closing begins under the influence of a force exerted by a return spring means.
Traditional peak and hold style fuel injectors use a single driving coil having a low resistance, fixed inductance, and fixed number of turns. During the opening phase, the ECU applies the system voltage (typically 12-14 volts) to the coil and the current builds quickly in the coil due to its low resistance. After the current inside the coil reaches a predetermined value, it may be lowered to a hold value by regulating the voltage. This arrangement allows for the magnetomotive force (MMF) in the injector to build to a high level very quickly, which results in a fast opening time, while minimizing the energy dissipated in the coil during the hold phase. The MMF is proportional to the magnitude of the current in the coil multiplied by the number of turns of the coil wire. Complex electronics are generally required to modulate the current in single coil injectors in order to achieve the low power dissipation described above because most ECUs are only equipped to provide a saturated switch driver and are not designed to modulate the current through single coil injectors in the above-described manner to achieve low power dissipation during the hold phase.
Dual coil injectors are known to reduce heat generation. Dual coil injectors typically have a low resistance primary stage and a high resistance secondary stage. The low resistance primary stage may be activated during the opening phase, resulting in a rapid current rise (due to the low DC resistance of the coil), and a corresponding rapid generation of a magnetic field within the coil. After a predetermined peak current value is reached in the coil, the high resistance secondary coil may be activated by placing it in series with the low resistance primary coil. Placing the coils in series has the desirable effect of increasing the effective DC resistance of the coil pair, and thus reducing the current through the windings and reducing the strength per turn of the resulting magnetic field. However, the added turns of the secondary coil also have the undesirable effect of contributing to the MMF acting on the armature during the hold phase.
Thus, even though the current is reduced, the total MMF acting on the armature during the hold phase is reduced only if the number of turns of the high resistance winding is kept to a minimum. This is because, while each additional turn results in increased resistance and corresponding decreased current in the winding, each turn also results in additional MMF acting on the armature. Accordingly, it is desirable to use wire having a high resistivity, such as brass wire, for the high resistance secondary winding in order to minimize the number of turns required to achieve the desired resistance. Copper, brass, and their alloys are typically used in fuel injector coil windings. Brass alloys may have two to four times the resistance of copper for the same cross sectional area. However, brass windings are often more difficult to manufacture and the wire is more expensive and less commonly available than corresponding sizes of copper wire. Even when wire with high resistivity is used, many active turns of the high resistance secondary coil may be required to achieve the desired resistance and corresponding reduction in current.
Additionally, because the effective inductance of the coil is proportional to the number of effective turns squared, the inductance of the injector increases as more turns are added. Because the closing time of the injector is dependent upon, among other factors, the effective inductance of the coil, it is desirable to minimize the effective inductance of the injector coil. Accordingly, there is a need for a highly efficient dual coil injector design having a fast response time and correspondingly low effective inductance.
The present invention provides a fuel injector with an armature and a means for biasing the armature toward a first position. A low resistance primary coil is wound in a first direction such that, when energized, it develops a magnetic force that opposes the biasing means and causes the armature to move from the first position to a second position. A high resistance secondary coil is positioned coaxially with the low resistance primary coil. The high resistance secondary coil has an at least partially reverse wound portion wound in a second direction opposite the first direction, such that the magnetic field generated by the at least partially reverse wound portion at least partially cancels the magnetic field of the low resistance primary coil.